1. Field of the Invention
This invention relates generally to a method and apparatus for performing elastographic diagnosis of a target body. Elastography is a system for measuring and imaging elastic modulus and compressibility distributions in an elastic tissue. It also has application to strain profiling and improved sonographic measurement and imaging. This system is typically based on external compression of a target body, and utilizes one or more transducers, acting as or with a compressor, to generate pre- and post-compression sonic pulses and receive the resulting echo sequences (A-lines) from within the target body. The pre- and post-compression echo sequence pairs may then be cross-correlated or matched to determine the strain along the path of the sonic pulses, and preferably to yield a strain profile of the target body. This strain profile may then be converted into a compressibility profile or elastogram by measuring the stress imposed by the compressing device and calculating the elastic moduli based on the stress and the strain profile.
An elastogram may be considered to be a special form of multi-trace sonogram, wherein each trace is a record or display with depth within a target body of an elastic modulus function of the body. A preferred elastic modulus function for purposes of display is the inverse of the bulk modulus, which provides a measure of compressibility. As explained later in this description, the inverse of the Young's moduli may usually be used instead of the bulk moduli. Less preferred but also helpful are the Young's moduli themselves. Methods for making and using elastograms are described at length in copending application Ser. No. 7/535,312.
While the methods described in application Ser. No. 7/535,312 produce greatly improved records and understanding of structures of elastic tissues, it has been observed that certain inaccuracies in the resulting elastograms may arise. In particular, inaccuracies have been observed if the transducer and compressor used to compress and insonify a tissue are relatively small in size relative to the depth (or thickness) of the target body, giving rise to decreasing stress in the target body as the distance increases from the compressor. Likewise, inaccuracies have been observed in elastograms and sonograms if the target body is not relatively homogeneous with respect to sonic speed, giving rise to strata through which sonic pulses travel at differing velocities. The improved methods and apparatus for elastography disclosed herein, while generally enhancing the accuracy of elastograms, have particular application in reducing the effect of such inaccuracies in both elastograms and sonograms.
2. Related Art
Traditional ultrasonic diagnosis is achieved by transmitting ultrasonic energy into a target body and generating an image from the resulting echo signals. A transducer is used to both transmit the ultrasonic energy and to receive the echo signals. During transmission, the transducer converts electrical energy into mechanical vibrations. Acquired echo signals produce mechanical oscillations in the transducer which are reconverted into electrical signals for amplification and recognition.
A plot or display (e.g., on an oscilloscope, etc.) of the electrical signal amplitude versus echo arrival time yields an amplitude line (A-line) or echo sequence corresponding to a particular ultrasonic transmission. When the A-line is displayed directly as a modulated sinusoidal pattern at radio frequency ("RF"), it is typically referred to as an RF or "undetected" signal. For imaging, the A-line is often demodulated to a non-RF or "detected" signal.
Ultrasound techniques have been extensively used in the field of diagnostic medicine as a non-invasive means of analyzing the properties of tissue in vivo (i.e., living). A human or animal body represents a nonhomogeneous medium for the propagation of ultrasound energy. Acoustic impedance changes at boundaries of regions having varying densities and/or sound speeds within such a target body. At such boundaries, a portion of the incident ultrasonic beam is reflected. Inhomogeneities within the tissue form lower level scatter sites that result in additional echo signals. Images may be generated from this information by modulating the intensities of pixels on a video display in proportion to the intensity of echo sequence segments from corresponding points within the target body.
Conventional imaging techniques are widely used to evaluate various diseases within organic tissue. Imaging provides information concerning the size, shape, and position of soft tissue structures using the assumption that sound velocity within the target is constant. Qualitative tissue characterization is carried out by interpretation of the grey scale appearance of the sonograms. Qualitative diagnosis largely depends on the skill and experience of the examiner as well as characteristics of the tissue. Images based only on relative tissue reflectivity, however, have limited used for quantitative assessment of disease states.
Techniques for quantitative tissue characterization using ultrasound are needed for more accurate diagnosis of disorders. In recent years many significant developments have been achieved in the field of ultrasonic tissue characterization. Some acoustic parameters, e.g., speed of sound and attenuation, have been successfully used for tissue characterization.
Tissue compressibility is an important parameter which is used to detect the presence of diffuse or localized disease. Measuring changes in compressibility becomes important in the analysis of tissue for pathological conditions. Many tumors are firmer than the surrounding normal tissue, and many diffuse diseases result in firmer or more tender pathology. Examples can be found in diffuse liver disease, prostate cancer, uterine fibroids, muscle conditioning or disease, breast cancer disease, and many other conditions.
Traditionally, physicians routinely palpate various regions of a patient's body to get an impression of tissue firmness or tissue softness. This technique is a form of remotely trying to sense what is going on in terms of tissue compliance. For example, in a liver, if the compliance in an area is sensed to be different from compliance in the surrounding area, the physician concludes from the tactile sensations in his fingers that something is wrong with the patient. The physician's fingers are used to perform a qualitative measurement.
In the last several years, a number of articles have appeared in the literature that explore various techniques for measurement and imaging of soft tissue compliance and tissue motion using ultrasound. These techniques rely on one of the following procedures: Doppler ultrasound velocity measurements, cross-correlation techniques to quantify motion in tissues, and visual inspection of M-mode and B-mode images. Additionally a Fourier feature extraction technique has been proposed. Internal mechanical excitation (motion of cardiac structures, arterial pulsation) or external vibration sources of motion produce displacement of the tissues under investigation. The displacements of different tissues are then analyzed by one of these techniques.
The amplitude and velocity of motion induced by arterial pulsation is generally too low for evaluation with Doppler velocity measurements. However, a number of researchers have used pulsed Doppler and color flow Doppler systems in conjunction with external mechanical harmonic excitations to determine the elastic properties of tissue. Using a low frequency external excitation source, the velocity of propagation of mechanical waves has been measured and relates to the modulus of elasticity of the tissues. The velocity of vibration of tissues under low frequency vibration excitation has been used to determine their relative compressibility. This technique has been termed "sonoelasticity" and produces B-scans which are "stained" with color coded relative compressibility information. Sophisticated Young's modulus measurements have been applied to determine muscle elasticity as a function of contractility state by measuring Doppler shifts due to very low frequency excitations (10 Hz). A similar approach using vibrations in the 100-1000 Hz range has been proposed to study dynamic muscle elasticity in vivo.
Cross-correlation techniques allow the use of either internally or externally generated sources of mechanical excitation due to their ability to quantify minute motions of tissue. External harmonic excitation has been used to assess motion of soft tissues with one dimensional and two dimensional correlators. The displacement and/or velocity of internally generated motion also have been measured using one dimensional and two dimensional correlators. Tissue strain caused by arterial pulsation in the liver and by transmitted cardiac motion in fetal lung have been proposed for tissue characterization.
Visual inspection of ultrasound M-mode waveforms has been used to study benign and malignant lesions in liver, pancreas and breast and to observe the elasticity of fetal lung. In magnified B-scans of the fetal thorax paracardiac lung movements have been measured to classify fetal lungs as stiff, intermediate or compliant. The examination of fetal lung sonograms has been used to evaluate compressibility as an indicator of lung tissue maturity.
But, one of the main difficulties in these methods is the lack of definition of the magnitude and direction of the driving force. This difficulty applies to driving forces that are internally generated by the pulsations of the heart and/or the aorta, as well as to those applied externally as low frequency and limited directivity. Further, it is difficult to measure the shape of an internal driving force, limiting the ability to determine how stress resulting from the driving force decreases as a function of distance from the driving force. The inability to define the direction, magnitude and shape of the driving force limits the ability of these methods to provide quantitative information about the elastic properties of the tissue under investigation.